Slavery was a darkest chapter in American history and I suspect the worst thing that ever happened in this country. Apart from moral and ethical aspect of it, the reality is slave labor is the least productive kind of labor out there. If the whites were half smart back then, they would have returned them all and saved the country from a century of tension and violence.
Sadly whites did not learn any lesson from slavery and the contemporary one’s in charge started doing the same thing by importing Hispanics and Asians and so on (This time they didn't call it slavery, they called it cheap labor!). So in essence history is repeating itself!